1. Field of the Invention
The invention relates to a method and apparatus for the prevention of high temperature corrosion due to alkali sulfates and chlorides in boilers and other fired apparatus.
2. Description of the Prior Art
The problem of high temperature corrosion in boilers and other types of fired apparatus has been appreciated for a very long time. Mainly because the process was not understood, however, attempts to find a solution were empirical or engineering make-do""s. There have been several feasible attempts to consider fuel additives (E. Raask, Mineral Impurities in Coal Combustion: Behavior, Problems and Remedial Measures, 1985) and such species as magnesium, calcium or silica based compounds were added to little avail. Instead, for the last 20 years or so, attention has turned to accepting the presence of high temperature corrosion, but minimizing its effect on materials by developing superalloys or using coatings.
Even more recently in the United States, the higher temperature gas turbine developers have given in altogether and now will only burn very clean fuels such as natural gas. Coatings and alloys have been a great help but are only a partial solution as they all degrade with time. Chan, in a paper entitled xe2x80x9cCoating Life Prediction Under Cyclic Oxidation Combustion,xe2x80x9d ASME J. Eng. Gas Turbines Power 120:609 (1998) formulates a coating lifetime model. What is more, all coatings and alloys must be prefabricated. They do not regenerate in use, but gradually degrade in an irreversible manner before finally failing. They generally are somewhat exotic and tough materials. These coatings are not flame components and it is not possible to create them in a normal combustion process. They are made by other techniques, prefabricated and then used. Coatings do not relate to flame chemistry or flame additives in any way. Consequently, patents relating to coatings or alloys are irrelevant to the invention.
The invention relates to flame deposition of alkali metal salts, primarily sodium and potassium salts, onto cooled metal surfaces immersed in the burnt gases. This has been one of the first quantitative studies and previously only qualitative engineering reports have been available. Nothing was previously known of the formation mechanisms or what were the controlling parameters. Some preliminary results are summarized in an earlier paper (26(trademark) International Symposium on Combustion, 1996) and in a more recent one (Combustion and Flame 129:453, (2002)). However, these papers do not discuss the claimed invention, but only discuss the background of the invention.
This invention is an extension to United States patent (Schofield, U.S. Pat. No. 6,328,911). The same technique is used but an additive of molybdenum salts rather than those of tungsten is invoked. The chemical interactions are very similar due to the close relationship between W and Mo in the periodic table.
An additive to a flame reaction is provided which forms noncorrosive deposits on cooler metal surfaces, which are more stable than Na2SO4 or K2SO4. The additive preferentially combines with the sodium or potassium, and sulfates of these are not formed. By adding molybdenum in the form of one of its salts to a flame, a polymolybdate is produced on the surface of a cooler metal object and corrosion is inhibited. Deposition appears to closely reflect the relative thermodynamic stabilities of these salts and follows the order, for example, Na2Mo4O13 greater than Na2Mo13O10 greater than Na2Mo2O7 greater than Na2MoO4 greater than Na2SO4 greater than NaCI  greater than Na2CO3. The flame sulfur or chlorine does not remain on the surface in association with the alkali metal. Molybdenum can be added in any form desired to the combustion system. The flame processes all chemical molybdenum salt precursors as equivalent sources of molybdenum. Suitable salts that are readily available are ammonium paramolybdate, molybdenum oxides, organo-molybdates or any other alkali-free molybdenum salt. The level of molybdenum needs to be twice or more that of the alkali in the specific flame environment based on atomic ratios. The nature of the fuel quality and possible alkali reduction schemes used in a practical combustor will control the quantity and mixing method of the additive. The additive may be directly mixed into the fuel or injected into the burned gas regions of the combustor. Molybdenum appears to be a viable alternative additive to tungsten.
The invention is thus a method of inhibiting corrosion in a flame reaction including an alkali metal comprising the steps of introducing an additive in the flame reaction, which additive forms a noncorrosive product with the alkali metal which is more chemically stable than sulfates of the alkali metal. The noncorrosive product is then deposited onto cooler metal structures in or proximate to the flame reaction in preference to or to the exclusive of more corrosive deposits which might otherwise be generated in the flame reaction.
The invention is also defined as the additive which is used for the foregoing method.
Generally, the alkali metal involved in the flame reaction is sodium (Na) or potassium (K). The additive includes molybdenum (Mo) in some form. When Mo is used, the noncorrosive product comprises a polymolybdate of the alkali metal.
The additive is preferably added to the flame reaction in a furnace, boiler, turbine or any combustion apparatus utilizing such fossil fuel. The additive may be added to the flame reaction in the form of ammonium paramolybdate, molybdenum oxides, organomolybdates or any other alkali-free molybdenum salt.
When the additive includes Mo, the amount of atomic Mo in the additive is at least 2 times as much as the amount of atomic metal alkali in the flame reaction.
The flame reaction is produced by combustion of a fuel. In one embodiment the additive is mixed in with the fuel prior to its combustion. In an other embodiment, the additive is injected into the combustion region or flame itself. Sulfur and chlorine can be included in the flame reaction without materially interfering with the desired end results.
While the apparatus and method has or will be described for the sake of grammatical fluidity with functional explanations, it is to be expressly understood that the claims, unless expressly formulated under 35 USC 112, are not to be construed as necessarily limited in any way by the construction of xe2x80x9cmeansxe2x80x9d or xe2x80x9cstepsxe2x80x9d limitations, but are to be accorded the full scope of the meaning and equivalents of the definition provided by the claims under the judicial doctrine of equivalents, and in the case where the claims are expressly formulated under 35 USC 112 are to be accorded full statutory equivalents under 35 USC 112. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals.